Variation Relationship among Multi-station Flood Series in Middle Reach of Xijiang River Basin

Xijiang River, the main stream of Pearl River Basin, suffered a lot from flood and the frequency and magnitude of flood increased obviously due to intensive human activities in recent years. In order to know the flood evolution law and its mechanism, hydrological variation diagnosis system was applied to analyze variation towards annual maximum peak flow and highest water level in middle reach of Xijiang River Basin. Hydrological variation diagnosis system is composed of three parts, namely primary diagnosis, detailed diagnosis and comprehensive diagnosis. It can solve the problems that single diagnosis method may give unreasonable result on occasion and the results of variant methods may be inconsistent. The results of variation diagnosis in Xijiang River shows that: annual maximum peak flow and highest water level mostly have medium jump at 1991 of Wuxuan, Dahuangjiangkou and Wuzhou station, which are located at mainstream. Except for a medium jump at 1993 of Taiping station, there are no variations or weak variations in tributary. The correlation analysis results indicate that water level closely relate to flood flow, thus flood flow variations lead to water level variations at mainstream. The consistent results of variations in mainstream and no variation in tributary reveal that the relationship of flood variation in mainstream are close, whereas floods in tributary have little influence on floods in mainstream. Analyses of physical causes show that climate change has little influence on flood variations in Xijiang River. Human activities have changed flood encounter of mainstream and tributary, laws of runoff yield and concentration, and flood returning channel. Thus, these are the main causes of flood variation in Xijiang River.